I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved method and apparatus for scheduling wake-up time in a mobile station in a slotted paging environment.
II. Description of the Related Art
The effort to efficiently transmit information to remote individuals can be satisfied in a variety of ways. Probably the most common remote communication device is the pager. Paging systems consist of a Public Switched Telephone Network (PSTN), a paging system controller, and a small number of transmitters broadcasting messages to remote paging receivers. To transmit a message to a pager a sender enters information through the PSTN. The sender does this by calling the pager telephone number and entering the information using the sender's telephone keypad. A sender may also use a computer with a modem to enter the information. The PSTN is linked to a paging system controller where the message entered by the sender is encoded such that the message will only be decoded by the intended pager. The encoded paging message is then sent to the transmitters for broadcast to the pager.
Paging systems allow a person to communicate in a limited manner to a remote pager. However, the information that can be transmitted to a pager is extremely limited. In the case of numeric pagers, the pager display can only show numeric messages. Typically only the sender's phone number is transmitted to the pager. The sender must rely on the paging recipient to call the phone number to communicate any more detailed information. The paging recipient may or may not be able to respond depending on the availability of a nearby telephone.
An improvement over the numeric pager is the alphanumeric pager. The alphanumeric pager is able to display alphabetic as well as numeric characters. This allows the sender to transmit a text message to the pager. The alphanumeric pager enables short messages to be transmitted that may not require any response from the paging recipient. When a reply is required from the paging recipient a phone number can be included in the message. To reply, the paging recipient would be required to call the sender as in the case of a numeric pager. Longer messages are more awkward since the pager display is limited to minimize the physical size of the pager. A user is required to scroll through a message that is longer than the pager's display length.
A disadvantage of the traditional paging system is that the paging communication link is one way. The pager can only notify a user of a received transmission and must continually await the receipt of a paging message. Pagers are merely receivers and do not provide any acknowledgement to the transmitting system that the message has been received. The paging network transmitters retransmit the message a limited number of times to increase the probability that the pager will receive the message. The paging network is also unable to determine the location of any pagers in the network. To overcome the inability to locate pagers within the system the paging network transmits all paging messages in all transmitters within the network.
Some of the disadvantages of a traditional paging system are overcome by the incorporation of a paging receiver within a wireless telephone. Nearly all wireless telephones incorporate an alphanumeric display. Short text messages can be transmitted to wireless phones in the same manner as would be transmitted to a pager. Where a user reply is requested, the recipient has ready access to a telephone. The need for the numeric message style page is essentially eliminated since the calling party could dial the number for the cellular phone rather than sending a page and waiting for the recipient to call back.
Particular implementations of paging within a wireless telephone network include those described in TIA IS-95, MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE SPREAD SPECTRUM CELLULAR SYSTEM, May 1995. Other Code Division Multiple Access (CDMA) wireless phones have implemented similar paging abilities.
The following description details some of the advancements and improvements gained under the CDMA implementation of paging within a wireless phone. In a CDMA wireless phone network a base station controller interfaces the Public Switched Telephone Network (PSTN) to the wireless network. The base station controller, in turn, links with multiple base station transceivers that control the wireless communication to the phone. When paging ability is implemented within a wireless phone the user no longer needs to carry two pieces of hardware. Additionally, the user is able to reply to short text messages using a readily available phone. However, improvements over the traditional paging system extend far beyond merely co-locating a pager with a wireless telephone. The use of CDMA is advantageous when integrating paging within a wireless phone. The use of CDMA allows the phone receiver to operate on one frequency and to distinguish between paging and traffic messages based on the code channel assigned to the message. Traffic messages may be voice or data messages. Specific codes correspond to Paging Channels and other codes correspond to Traffic Channels. Using codes rather than unique frequencies eliminates the need to actively modify the tuning frequency of the receiver to correspond to paging or traffic channels.
The wireless phone's ability to transmit messages provides major improvements to the paging network that may not be obvious to the user. The transmitter portion of the phone allows the phone to acknowledge when a paging message has been received. If acknowledgement is requested by the base station the phone is able to transmit an acknowledge message and send it to the base station using an Access Channel. The phone's ability to acknowledge messages decreases the base station's need to repeatedly retransmit paging messages. Eliminating the base station's need to repeatedly retransmit paging messages increases the capacity of the paging system.
Additionally, a CDMA phone periodically registers with the base station transceiver that is providing coverage in the area the phone resides. When the CDMA phone registers, the base station controller is provided information as to the general location of the phone. When paging messages are generated for a particular phone the paging system is not required to transmit the message from all transmitters but can limit the transmission to the base station transceivers that are within the vicinity of the last phone registration. Limiting the transmissions results in an increase in the capacity of the paging system since not all transmitters are required to transmit the same message at the same time.
The wireless phone operates primarily from battery power. Extending battery life maximizes the talk times and standby times of the phone. To conserve battery power the CDMA phone can be instructed to only respond to pages during specific time periods. During time periods when the phone is not required to actively monitor the Paging Channel the phone can power down into a sleep mode to conserve power. In slotted paging the phone is assigned specific time periods where the phone must actively monitor the Paging Channel. In the CDMA system described in TIA IS-95, the slotted paging structure is defined as having a maximum slot cycle consisting of 2048 paging slots of 80 milliseconds in length. The phone is able to inform the base station that it will operate in slotted mode. The wireless phone tells the base station what slot cycle timing it will be using. The wireless phone then monitors the Paging Channel during the assigned slot cycle. In general, slotted paging refers to any division of a maximum slot cycle into sub-slots where the phone is only required to actively monitor the sub-slot assigned to it. During the periods in which the phone is not monitoring the Paging Channel the phone decreases operation to a sleep level to maximize power conservation.
Although the use of slotted paging allows the wireless phone to conserve power the phone still must monitor every slot assigned to it. This cyclic monitoring of the Paging Channel can be improved to further minimize power consumption within the phone. The likelihood of any particular phone receiving a page in a specific paging slot cycle is probabilistic. Moreover, the probability of receiving a page within any paging slot cycle is a random process that varies with time. What is required is a method of making wake-up decisions within the phone that allows the phone to skip assigned paging slots. The decision to skip assigned paging slots must ensure that the probability of a missed page is low.